In orthopedics, successful treatment of nonunion and fracture of small bone fragments often involves the use of small screws. These screws are installed through both fragments in a direction substantially normal to the fracture plane. The small screw must compress the fragments sufficiently to promote healing.
Fractures resulting in small bone fragments often occur in bones adjoining intra-articular regions, such as the scaphoid, other carpal bones, ends of long bones, and bones of the spine. It is important in these applications that the head of the bone screw does not protrude from the outer surface of the bone, where the screw head could interfere with joint movement or damage surrounding tissue during articulation.
Standard small bone screws having threads only on the leading portion of the shank have been tried in this application. These screws are threaded into pre-drilled holes until the head contacts the outer fragment. The screw is then turned until a force is generated compressing the fragments between the screw head and the threads, which engage the inner fragment. In intra-articular applications, the pre-drilled hole must be countersunk to prevent the head of the screw from protruding from the bone surface. The depth of this counterbore often exceeds the thickness of the relatively thin bone cortex and penetrates to the softer cancellous bone below. When tightened, the small screw head can sink further into the cancellous bone, providing insufficient compressive forces for proper union of the fragments.
Another approach to this problem is disclosed in U.S. Pat. No. 4,175,555 to Herbert, which shows a small, headless screw utilizing threads of different pitch at the leading and trailing ends. The differential in pitch draws the bone fragments together as the screw is tightened. Because the screw head is replaced by the threads on the trailing end, the screw does not protrude from the bone and does not require countersinking. The threads on the trailing portion of the screw provide some purchase in the cancellous bone material. These screws, however, provide insufficient compression in many applications. Furthermore, a surgeon installing the screw must exercise care in starting each set of threads into the bone so that proper fracture reduction and compression occur simultaneously with the screw head reaching the proper depth. Otherwise, optimum fracture compression may occur too early, with the trailing end protruding from the bone, or too late, with the trailing screw threads engaging part of the inner fragment.
U.S. Pat. No. Re. 33,348 to Lower discloses a hip screw with an unthreaded shaft section and a leading threaded portion. A threaded sleeve member is retained on and slidable along the unthreaded shaft portion. The threads of the sleeve are designed to engage the relatively thick bone cortex of the femur. The threads on the leading portion and on the sleeve have different pitches. The assembled screw is installed as a unit, with compressive forces being generated as a result of the pitch differential as in Herbert '555. The two-piece design permits the unthreaded shaft section of the screw to back out through the sleeve member when bone absorption occurs, solving the problem of prior hip screws being forced in the opposite direction, into the hip joint capsule.
A method frequently used for the installation of small bone screws is to first install a guide wire in the bone in the location and orientation decided upon for the screw. The wire passes through the surrounding soft tissue, providing a guide for tools, such as screwdrivers, reamers and drills, and a guide for the orthopedic implants, such as screws and washers. Tools and implants used in such a procedure are cannulated, i.e. provided with a central bore through the long axis for placement over the wire.